1. Field of the Invention
The present invention relates to a robot control device for controlling a motor-driven robot.
2. Description of the Related Art
Conventionally, robots are controlled by feeding back only the angular velocity, rotation angle, and torque (current value) of a motor, for example, a servomotor. In such a control method, it is only required to prepare only an encoder or an electric current detector attached to the motor.
However, in recent years, robots have been used in various fields, and conventional control methods may be insufficient. Thus, in addition to an encoder or an electric current detector, another sensor has been used to control a robot.
In, for example, Japanese Unexamined Patent Publication (Kokai) No. 2012-168926, full-closed position control using an additional encoder is performed to reduce a position error caused by, for example, a transmission error of a decelerator. Further, in Japanese Unexamined Patent Publication (Kokai) No. 2011-136416, position control using a position-measuring instrument, such as a laser tracker or indoor GPS, is performed to accurately position an aircraft fuselage and other parts supported by a robot.
However, originally, a range, in which a robot is movable, does not necessarily coincide with a range, in which an additional sensor is applicable. Thus, a change of the control method, in which an additional sensor is used, may cause limitation of the operating range of the robot.
In an example, application of full-closed control to accurately operate a robot will be discussed below. In this respect, an angle encoder or a rotary encoder is used as an additional sensor. However, in order to attach such an additional sensor to the robot, it is necessary to drastically change the design of the robot. Thus, for economic reasons, a tape-type linear scale may be attached to an existing robot to perform full-closed control.
However, when the tape-type linear scale is used, a signal is interrupted at an end point of a tape. Thus, when the full-closed control is used, a rotary shaft would not rotate more than 360 degrees if the rotary shaft can rotate more than 360 degrees.
In another example, a control operation, in which a laser tracker is used to feed back the position and posture of an end effector of a robot, to accurately operate the robot, will be discussed below. In order to measure the position and posture using the laser tracker, it is necessary that a reflector attached to a robot or a tool of the robot can be seen from the tracker disposed around the robot.
However, depending on the posture of the robot, the reflector may not be seen due to a peripheral device, an arm of the robot, or a tool attached to the robot, which functions as an obstacle. Accordingly, a control operation using a laser tracker limits the operating range of the robot.
Thus, control systems may be switched so that a control operation using an additional sensor is performed when the additional sensor is necessary, and a control operation without an additional sensor is performed when the additional sensor is not necessary. Therefore, the original operating range of the robot is maintained.
In this respect, when the control systems are rapidly switched, a mechanical impact may occur, and accordingly, it is preferable that the control systems are smoothly switched. Further, when the control systems are switched in accordance with an operation program, the complexity of the grogram increases as the frequency of switching increases. Thus, it is preferable that the control systems are automatically switched.
Japanese Unexamined Patent Publication (Kokai) No. 2001-222324 discloses that the ratio between semi-closed control without an additional sensor and semi-closed control using an additional sensor, which are applied to position control, is changed by switching control gains, so that control systems are smoothly switched.
Japanese Unexamined Patent Publication (Kokai) No. 05-158542 discloses that control systems are switched depending on the property of an object to be controlled. Assuming that the position and posture of a robot are included in the property of an object to be controlled, the control systems can be switched depending on the position and posture of the robot.